Fuel cells are generally made up of a cathode in its compartment and an anode in its compartment separated by an ion-exchange membrane that is usually a fluoropolymer having acid groups, e.g. sulfonic acid groups attached to the polymer chain. These are referred to as sulfonic acid ionomers, or, in membrane form, as sulfonic acid membranes. A fuel such as hydrogen or methanol is fed to the anode compartment, where it is oxidized, generating protons, which pass through the membrane and combine with oxygen at the cathode, producing water.
The management of water in the fuel cell, particularly control of water passing from the anode to the cathode side of the membrane, is important, because membrane conductivity is a function of water content and excessive water transport can result in drying out of the anode side of the membrane. In addition water accumulation at the cathode can result in flooding of the cathode, which interferes with cathode functioning and reduces fuel cell performance.
Existing fuel cell sulfonic acid membranes often have higher water transport than is desirable for best operation.
Direct methanol fuel cells (DMFC) use methanol as fuel directly, i.e. without first reforming the methanol to produce hydrogen to be used in the cell, typically use about 1-6% solution of methanol in water. Permeation of methanol through the membrane that separates the anode from the cathode is a problem in direct methanol fuel cells. This is referred to as methanol crossover. Crossover represents a loss of fuel cell efficiency because the crossover methanol is consumed without producing electric current. This direct consumption of methanol at the cathode also adversely affects the functioning of the cathode. Reduction of methanol crossover in direct methanol fuel cells is needed.
Membranes containing carboxylic acid ionomer, present as a layer laminated or coextruded with a layer of sulfonic acid ionomer, are widely used in the chloralkali industry as membrane separators in cells for the electrolysis of brine because of their superior ability to reject anions. These membranes, in chloralkali electrolyzers have lower water transport properties than do sulfonic acid ionomer membranes. However, though carboxylic acid ionomer membranes show acceptable conductivity in the chloralkali environment, where sodium or potassium ions conduct the current through the membrane, their conductivity when protons are the current carriers, as in the case of fuel cells, is very low. It is generally believed that carboxylic acid ionomer membranes are not useful in fuel cells.